This invention relates to the preparation of long persistence alkaline earth sulfide phosphors. More particularly, this invention relates to a method of making alkaline earth sulfide phosphors using two firing steps to improve the persistence time.
Long persistence phosphors that emit in the green, blue-green and blue colors have been known for some time. However, orange-red emitting long persistence phosphors have been discovered only recently. For example, Royce et al, U.S. Pat. No. 5,650,094 have disclosed rare earth activated divalent titanate phosphors, such as CaTiO3 or Caxe2x80x94Znxe2x80x94Mgxe2x80x94TiO3 that emit in the red portion of the spectrum, but the emission is visible for only a few minutes. Lindmayer, U.S. Pat. No. 5,043,096, reported a red-emitting alkaline earth metal sulfide based phosphor doped with two or more rare earths in the form of their oxides, and then fluxed with a halide, such as LiF. However, the resultant fired fluoride phosphor was highly sintered, and had to be ground to obtain a useful powdered phosphor. However, grinding degrades the emission, and thus the phosphor must be heated or annealed to repair the crystal defect damage. However, the emission performance is never fully restored. These phosphors are described as being useful as additives to paint formulations.
Other known long persistence alkaline earth metal sulfide phosphors that emit in the red, such as CaS:Eu:Tm have a short decay time of about 30 minutes.
Red and orange-emitting phosphors are highly desirable because they are easy to see in the dark, particularly in the event of a power failures, to show xe2x80x9cEXITxe2x80x9d signs in a darkened room, and to make visible other safety devices and the like. Since red and orange are desirable, bright colors, they are also sought after for toys, sporting goods and the like.
We have found that alkaline earth sulfide phosphors doped with divalent europium, a halide ion and a controlled amount of an oxide ion, has long persistence and emits in the orange and red spectral regions.
These phosphors can be prepared by firing a mixture of alkaline earth sulfates activated with europium, one or more trivalent rare earth metal cations, a fluxing agent, a hydrocarbon and sulfur in a closed refractory crucible and heated to an initial temperature of about 300-400xc2x0 C. to react the hydrocarbon and sulfur to form hydrogen sulfide and carbon; the temperature is then raised to about 900-1200xc2x0 C. when the remaining carbon reacts with the alkaline earth sulfate to form the corresponding sulfide. Trivalent rare earths can be added to the mixture, and a halide is also added, either in the form of an alkaline earth halide or as an ammonium halide. All of the ingredients are then fired together.
Alternately, the halide can be added during a second firing step after the initial fired product has been ground. The second firing results in a less sintered product which requires less grinding to form a finely divided phosphor, and therefore has improved luminescent properties.
The long persistence orange and red phosphors of the invention are prepared from alkaline earth metal sulfides, such as CaS, SrS and BaS, activated with europium (Eu). Both oxide and halide ions are also present. A trivalent rare earth ion, including erbium (Er), praseodymium (Pr), holmium (Ho) dysprosium (Dy), gadolinium (Gd), terbium (Tb) or neodymium (Nd) can also be added to regulate the amount of oxide retained in the host lattice by a strong bonding attraction between the trivalent ion and the oxide ion. This attraction prevents the reduction of the oxide ion concentration to levels that are too low to achieve long persistence in the phosphor emission.
These phosphors are prepared by forming a mixture of an alkaline earth metal sulfate, such as SrSO4 doped with Eu, one or more trivalent rare earth metal ions including Er, Ho, Dy and Nd, to form a doped alkaline earth metal sulfate. This doped alkaline earth metal sulfate is mixed with a powdered hydrocarbon and powdered sulfur and fired.
Suitable hydrocarbons include polyethylene, polypropylene, paraffin or mineral oil.
The mixture is placed in a refractory crucible, such as one made of alumina, which is covered. The covered crucible is then heated slowly up to a temperature range of 300-400xc2x0 C., when sulfur reacts with the hydrocarbon to produce hydrogen sulfide and carbon in accordance with the equation
CH2+Sxe2x86x92C+H2S
The temperature is increased to a range of 900-1200xc2x0 C. when free carbon reacts with the alkaline earth sulfate to form the corresponding sulfide, in accordance with the equation
SrSO4:Eu:Tr+3CH2+3Sxe2x86x92SrS:Eu:Tr+2CO+CO2+3H2S
wherein Tr is a trivalent rare earth metal ion. This firing can be carried out for from about 0.5 to 4 hours.
The halide, which can be fluoride, chloride, bromide or iodide, can be added in several ways; for example, an ammonium halide, such as ammonium chloride, ammonium bromide or ammonium iodide, or an alkaline earth metal halide, such as strontium chloride or barium chloride, can be added to the mixture and fired. However, since this results in a sintered mass which must be ground prior to use, preferably the halide is added in a second firing step, after grinding the doped alkaline earth metal sulfide product of the first firing. The second firing results in a material that is less sintered, which requires less grinding that reduces luminescent properties.
The degree of sintering of the phosphor depends on the firing temperature; the lower the temperature, the less sintering occurs.
To achieve a higher degree of persistence, a second firing is carried out at the same temperatures, but using less hydrocarbon and sulfur, and adding a halide compound such as ammonium chloride. The firing regime can be the same for both firings. The resultant phosphor has long persistence, i.e., up to two hours, and emits in the orange portion of the spectrum. This phosphor has the formula
AS:Eum:Trn:OxXy
wherein A is an alkaline earth metal ion; Tr is one or more trivalent rare earth metal ions; X is a halide; m is an integer of 0.01 to 0.5 atomic percent; n is an integer of from 0.03 to 0.5 atomic percent; x is an integer of from 0.01 to 2.0 atomic percent; and y is an integer of from 0.01 to 0.5 atomic percent.